This invention relates to tunable lasers, and more specifically relates to an improved apparatus and method for tuning a laser having a tunable laser cavity.
It is known to employ a birefringent filter as the tuning element in a tunable laser. Tuning of birefringent filter elements has previously been accomplished mechanically as well as electro-optically. One prior art tuning technique utilizes an intracavity grating and a single piece of electro-optical crystal between crossed polarizers. Using this technique, tuning ranges in the order of a few angstroms and tuning rates of a few tenths of an angstrom per kilovolt of applied voltage have been obtained. In order to obtain a larger tuning rate, two suitably oriented crystals with a precisely specified length difference and a stack of Brewster angle plates have been employed. Both of these prior art techniques require the use of a relatively high-gain pulsed laser, and neither technique provides a simple yet precise, sensitive and broadbanded laser tuning device.
U.S. Pat. No. 3,414,839 to Bridges et al discloses a unidirectional ring laser employing a tunable dispersive loss element which is shown in one embodiment as an etalon interposed at nearly normal incidence in the beam path to provide a variation of optical loss for optical transmittance with respect to frequency. A laser modulation device having means for injecting an optical bias by inclining a reflection mirror to create an angular offset between the optical axis of a crystal and the laser optical path is shown in Japanese Patent Publication No. Sho 47/1972-33558, by Goto, published Aug. 25, 1972. Neither of these references shows nor suggests the novel laser tuning apparatus and method disclosed herein.
The lack of a broad-banded laser tuning device which can be simply yet precisely and accurately tuned over a wide range has hampered the development of tunable laser systems for transient and derivative spectroscopy. While the general concept of incorporating a tunable laser light source into a spectroscopy system is known, as illustrated by U.S. Pat. No. 3,588,253 to Wittmann and U.S. Pat. No. 3,805,074 to McCormack, these prior art systems do not offer the unique advantages obtainable with a broad-banded, precisely tunable laser source.